Many conventional oil wells are operated by a downhole pump at or close to the bottom of the well, the pump being of a reciprocating kind and actuated by a rod string which in turn is reciprocated vertically by a pump jack. Recently, rotary drive progressive cavity pumps have been installed to replace the pump jack vertically actuated reciprocating pumps. Such rotary pumps are particularly suited for the production of crude oil laden with sand and water.
In the earlier construction, using simple reciprocation to operate the pump, the string is secured to the pump jack by the intermediary of a two-piece clamp secured around the top end of the rod string, and tied to the pump jack by wires or the like. Although such clamps were adequate for the simple vertical reciprocation of the rod string, they operate close to the limit when required to transmit torque to the string as well as to support the weight of the string and the downhole rotor.
At high torque values, the conventional clamp tends to slip with respect to the drive string, allowing the latter to move downwardly. For this reason, a backup clamp is often mounted behind (above) the pump jack clamp. This means that the drive string cannot be mounted at the bottom position, since some margin must be left for slippage. The typical approach has been to raise the drive string by approximately one foot, to allow for possible slippage. However, if the pump rotor is raised or lowered after slippage with respect to the pump stator at the bottom of the well, the full capacity of the pump is not available. In order to use the full capacity of such a pump, a positive position lock and drive system must be utilized, i.e. one in which there is no risk that the rod string will slip downwardly.